Laminated ceramic capacitors produced by simultaneous sintering of dielectric ceramic composition and internal electrodes have been developed in recent years as compact, large-capacity capacitor elements. Traditional dielectric ceramic compositions have high sintering temperatures of 1150° C. to 1400° C. and therefore nickel (Ni) or nickel alloy that withstands high temperatures has been used as the mainstream electrode material for simultaneous sintering with internal electrodes. However, nickel is a rare metal and, as the demand for rare metals is expected to grow, alternative technologies are drawing the attention of late and there is a growing demand to replace nickel with more affordable copper (Cu).
On the other hand, since copper has a melting point of 1085° C., which is lower than the melting point of nickel, use of copper in internal electrodes requires sintering to be implemented at 1030° C. or preferably 1000° C. or below, which gives rise to a need for dielectric material for laminated ceramic capacitors that can demonstrate sufficient characteristics even when sintered at temperatures lower than the temperatures traditionally used. The inventions described below are known as dielectric ceramic compositions meeting the aforementioned requirements, and these inventions describe use of copper in internal electrodes for laminated ceramic capacitors.
For example, Patent Literature 1 describes a dielectric ceramic composition expressed by the composition formula 100 (Ba1-xCax)mTiO3+aMnO+bV2O5+cSiO2+dRe2O3 (where Re represents at least one type of metal element selected from Y, La, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb, and a, b, c and d represent mol ratios), wherein such dielectric ceramic composition is characterized by the ranges of 0.030≦x≦0.20, 0.990≦m≦1.030, 0.010≦a≦5.0, 0.050≦b≦2.5, 0.20≦c≦8.0, and 0.050≦d≦2.5. This dielectric ceramic composition is described as follows: “A dielectric ceramic composition can be obtained which has a relative dielectric constant of 3000 or more, but the absolute rate of change in DC bias when 2 V/μm of DC voltage is applied is small at 20% or less, even after the dielectric ceramic layer is made as thin as approx. 1 μm; which has flat temperature characteristics of relative dielectric constant to satisfy the X7R characteristics under the EIA standard (the absolute rate of change in relative dielectric constant due to temperature in a range of −55° C. to 125° C. is within 15% of the reference relative dielectric constant at 25° C.); which has high insulation resistance as represented by a resistivity of 10−11 Ω·m or more at 25° C.; and which has high high-temperature load reliability as represented by an average time to failure of 100 hours or more when a DC voltage of 10 V/μm in electric field intensity is applied at 150° C.” (Paragraph [0018]) and “The internal electrodes are constituted by at least one type of conductive material selected from Ni, Ni alloy, Cu and Cu alloy” (Paragraph [0017]). However, the sintering temperature of the obtained dielectric ceramic composition is still high at 1100° C. or above and in reality the internal electrodes cannot be sintered in a stable manner using copper, and therefore further improvement is needed by lowering the sintering temperature.
In addition, Patent Literature 2 describes a reduction-resistant dielectric ceramic composition whose primary component is expressed by the composition formula α (SrXCaYBa1-X-Y) (Ti1-WMW)O3+(1−α)((Bi1-Zn*AZ)2O3+βTiO2) (where M represents at least one type of element selected from Zr and Mg, and A represents at least one type of element selected from Li, K, and Na), wherein the mol ratio α of (SrXCaYBa1-X-Y)(Ti1-WMW)O3 relative to the primary component and mol ratio β of Ti per 1 mol of (Bi1-Zn*AZ)2O3 are in the ranges of 0.60<α<0.85 and 1.5<β<4.0. This reduction-resistant dielectric ceramic composition is described as follows: “Provide a reduction-resistant dielectric ceramic composition which can offer an improved relative dielectric constant without containing lead or other substance harmful to the environment or to the human body in its composition and also a lower rate of change in relative dielectric constant due to temperature, to meet the need for a reduction-resistant dielectric ceramic composition supporting Cu electrodes and other base metal electrodes” (Paragraph [0009]) and “A reduction-resistance dielectric ceramic composition can be obtained which is free of lead in its composition, has a high dielectric constant of 1000 or more, offers excellent temperature characteristics as represented by a rate of change in relative dielectric constant due to temperature of within ±10% at temperatures between −25° C. and +85° C., and exhibits excellent insulation property even after reductive sintering as represented by a CR product of capacitance and insulation resistance of 1000 MΩμF or more” (Paragraph [0013]). However, the reduction-resistant dielectric ceramic composition in Patent Literature 2 uses bismuth, which is a heavy metal, instead of lead, and there is no mention of bismuth-free reduction-resistant dielectric ceramic composition.
Furthermore, Patent Literature 3 describes a dielectric ceramic composition characterized in that it contains a BaO—TiO2—ReO3/2 ceramic composition expressed by xBaO-yTiO2-zReO3/2 (where x, y, and z represent percents by mol in the ranges of 8≦x≦18, 52.5≦y≦65 and 20≦z≦40, and x+y+z=100 and Re is a rare earth element) as well as a glass composition containing 10 to 25 percent by weight of SiO2, 10 to 40 percent by weight of B2O3, 25 to 55 percent by weight of MgO, 0 to 20 percent by weight of ZnO, 0 to 15 percent by weight of Al2O3, 0.5 to 10 percent by weight of Li2O, and 0 to 10 percent by weight of RO (where R represents at least one type of element selected from Ba, Sr, and Ca), which is described as follows: “Can be sintered at low temperatures of 1000° C. or below and also co-sintered with Ag, Au, Cu or other metal offering excellent electrical conductivity” (Paragraph [0016]). However, the obtained dielectric ceramic composition has a relative dielectric constant of 50 or less and thus is not suitable as a dielectric material for compact, large-capacity laminated ceramic capacitors.
In light of the aforementioned situation, the inventor of the present invention studied with the aim of obtaining a laminated ceramic capacitor which can be sintered at 1080° C. or below in a reducing ambience, which does not contain lead (Pb) or bismuth (Bi) in its dielectric ceramic layers, and which has a dielectric constant of 2000 or more, X7R temperature characteristics of dielectric constant and high-temperature stress longevity traits equivalent to conventional laminated ceramic capacitors with Ni internal electrodes, and discovered conditions for Ba/Ti ratio, composition ratio of rare earths as auxiliary components, and MnO composition ratio, for a dielectric ceramic composition whose primary component is a BaTiO3 compound. Based on the above, the inventor of the present invention proposes a laminated ceramic capacitor having: multiple dielectric ceramic layers; internal electrodes which are formed between the dielectric ceramic layers in a manner opposing each other and led out alternately to different end faces; and external electrodes which are formed on both end faces of the dielectric ceramic layers and each connected electrically to the internal electrodes; wherein such laminated ceramic capacitor is characterized in that: the dielectric ceramic layer is a sintered compact constituted by a primary component expressed by ABO3+aRe2O3+bMnO (where ABO3 is a perovskite dielectric mainly constituted by BaTiO3, Re2O3 represents at least one type of metal oxide selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y, and a and b represent mol numbers relative to 100 mol of ABO3) in the ranges of 1.000≦A/B≦1.035, 0.05≦a≦0.75, and 0.25≦b≦2.0, as well as auxiliary components that include at least one type of element selected from B, Li, and Si for a total of 0.16 to 1.6 parts by mass in equivalent B2O3, Li2O, and SiO2, respectively; and that the internal electrodes are constituted by Cu or Cu alloy (Patent Literature 4). The inventor of the present invention also proposes that, with a laminated ceramic capacitor whose internal electrodes are constituted by Cu or Cu alloy, X7R or X8R temperature characteristics can be achieved by obtaining its dielectric ceramic as a sintered compact of perovskite dielectric material primarily constituted by BaTiO3, comprised of grains whose average diameter is 400 nm or less in a section view as well as grain boundaries (Patent Literature 5), where examples illustrate mixtures of MnO as a starting material for sintered compact, with B2O3, Li2O, and SiO2 as additive rare earth oxides and sintering auxiliaries.